A wildly popular way of chorusing over a jazz song. Four / four basically means alternating every four bars of standard soloing, collective improvisation or whatnot with a four-bar drum fill.
Usually, this dialogue of melody/harmony and percussion works itself up to some kind of climax at the end of the chorus, at which point it's a good idea to go back to the theme or to flat-out go for a drum solo.
A nice song to do a lot of sophisticated drumming, and thus also a nice song to do four / four, is Tizol/Ellington's Caravan, standard of standards. You just go through an entire chorus (i.e. 2x part A, 1x part B) playing the basic groove, no theme, no soloing, no drums, for four bars. Then you shut up for the next four bars, while your drummer really swings it. And again and again and again: you groove, then the drummer continues going berzerk.
Wholesome fun for the family, I say.
When first noded, this writeup was killed because of the misleading node title. The responsible god has, however, apologised by now and encouraged me to re-node this.